As generally practiced, in the manufacturing process of a stator that is an automotive electrical generator, a three-phase stator coil is wound around a circular core, and after that, the entire stator coil including a circular winding coil base that is sticking out of an end face of the core in the axial direction thereof, is impregnated with varnish. In this state of things, the stator coil is heated, and thereby the impregnating varnish is harden-dried.
After impregnating the stator coil with varnish, it is conventionally practiced that the entire stator is inserted into a heating furnace while being rotated, and thereby the impregnating varnish is harden-dried with heated air.
However, in the method of harden-drying varnish by using a heating furnace, a temperature of the stator is increased in a slow manner after the stator is inserted into a heating furnace. Therefore, in order to satisfy enough thermal and temporal conditions to harden varnish material, a longer heating time is required and more electrical energy is consumed, which causes high costs. Furthermore, in order to keep a constant temperature in a heating furnace, a temperature in a heating furnace is required to be increased before inserting a workpiece therein and is required to be kept all the while until entirely discharging a workpiece therefrom, which makes costs still higher.
Meanwhile, in the process of preheating the stator coil before impregnating the stator coil with varnish it is conventionally suggested for the purpose of improving operating efficiency and also saving electrical energy by reduction of a heating time, that a high-frequency induction heating coil head is positioned in the central opening of the circular core, then the stator coil and the core are heated rapidly by the high-frequency induction heating method.
A heating means using this above-mentioned high-frequency induction heating coil head is probatively introduced to the process of harden-drying varnish impregnating the stator coil. Concretely, a high-frequency induction heating coil head is set from outside in the thickness direction of the core of the stator, against the winding coil base that is sticking out of each of the end faces of the core in the thickness direction (the axial direction) thereof, and then the stator coil is heated with high-frequency induction heat applied by the high-frequency induction heating coil head.
Furthermore, it is conventionally suggested that the stator coil is energized to cause self-heating, and thus the stator coil is heated (as referred to Patent Document 1: Japanese Unexamined Laid-open Patent Publication No. S60-82050, for example). However, in the case of heating the stator coil by employing the induction heating method, a part of a magnetic flux generated by an induction heating coil head acts on the end faces of the core in the thickness direction thereof. Since a core is usually constructed of layered silicon steel plates and the thermal conductivity in the thickness direction of such a core is low, only the end faces of the core in the thickness direction thereof are particularly heated, and the end faces of the core in the thickness direction thereof could be thermally deformed. And accordingly, the stator coil could not be heated favorably, which leaves a problem.
As for the method of energizing the stator coil to cause self-heating, there is a tendency that a self-heating temperature is varied depending on a type of the stator coil. Therefore, this method is not an easy choice and leaves a problem.
On the other hand, in the manufacturing process of a stator such as an automotive electrical generator, a stator coil wound around the inner circumference of a circular core is sometimes impregnated with varnish or plastic-molded. In these cases, it is necessary to heat the stator coil and the core before varnish impregnation or plastic-mold processing.
Conventionally, the entire stator coil is heated in a hot-air heating furnace before varnish impregnation or plastic-mold processing.
However, in the conventional method above, since the stator coil is heated from its surface in a slow manner, a longer heating time is required. Accordingly, more electrical energy is consumed.
And even trying to heat the stator coil and the core uniformly and evenly, a long heating time is required and keeping the stator coil at a predetermined temperature is not easy. As results, varnish applied to the stator coil could not be impregnated well and the molding could lose its quality because a crack or etc. could be caused, which leaves a problem.
To cope therewith, it is suggested for the purpose of improving temperature variation in the stator, saving electrical energy by reduction of a heating time, and etc., that a high-frequency induction heating coil head is positioned in the central opening of the circular core, and thereby the stator coil and the core are heated rapidly by the high-frequency induction heating method.
And also, as described in Patent Document 1 above, it is also suggested that the stator coil is heated more uniformly by energization to cause self-heating in addition to the high-frequency induction heating method.
However, in the method of positioning a high-frequency induction heating coil head in the central opening of the circular core and heating the stator coil and the core by the high-frequency induction heating method, a large part of a magnetic flux generated by the high-frequency induction heating coil head acts on the outer circumference areas of the end faces of the core in the thickness direction thereof. Since a core is usually constructed of layered silicon steel plates and the thermal conductivity in the thickness direction of such a core is low, only these parts are particularly heated and thereby thermal deformation could be caused and its insulating resin could be damaged, and thus the stator coil could not be heated favorably, which leaves a problem.
And in the method of heating the stator coil and the core by energization to cause self-heating in addition to the high-frequency induction heating method, a self-heating temperature is varied depending on a type of the stator coil, and the entire stator coil could be vibrated by an electromagnetic force generated when the stator coil is energized, which leaves a problem. Furthermore, it is troublesome to determine a position of an energization terminal and automation is not easy, and sparks could be occurred between connection terminals due to poorly fitting contacts when the stator coil and a power source for energization are connected, which leaves a problem.